1. Field of the Invention
The present invention relates to a magnetic recording medium and, particularly, to a magnetic recording medium provided with a magnetic layer containing metal nanoparticles having hard magnetism.
2. Description of the Related Art
In order to improve magnetic recording density, it is necessary to decrease a particle size of a magnetic substance contained in a magnetic layer. For example, when a magnetic recording medium widely used as a video tape, a computer tape or a disk is compared with another one having a ferromagnetic body of the same weight, the smaller the particle size is, the lower the level of noise is.
CuAu type or Cu3Au type magnetically hard ordered alloys (superlattice type magnet) have large magnetic anisotropy of crystal due to the strain produced when these alloys are ordered and therefore have hard magnetism even when a particle size thereof is decreased to prepare so-called metal nanoparticles. These alloys are therefore promising materials for improving magnetic recording density.
Examples of a method of synthesizing metal nanoparticles which can form CuAu type or Cu3Au type alloys include (1) an alcohol reduction method using a primary alcohol, (2) a polyol reduction method using a secondary, tertiary, dihydric or trihydric alcohol, (3) a thermal decomposition method, (4) an ultrasonic decomposition method, (5) a reduction method using a strong reducing agent, and the like when classified according to precipitation methods (for example, Japanese Patent Application Laid-Open (JP-A) No. 2003-73705).
Also, when classified according to reaction systems, examples of a method of synthesizing metal nanoparticles which can form CuAu type or Cu3Au type alloys include (6) a polymer existence method, (7) a high-boiling point solvent method, (8) a normal micelle method, (9) a reverse micelle method and the like.
The metal nanoparticles synthesized by the above-described methods have a face-centered cubic crystal structure. A face-centered cubic crystal generally has soft magnetism or paramagnetism. Crystals having soft magnetism or paramagnetism are not suitable for recording media. In order to obtain a magnetically hard ordered alloy having a coercive force of at least 95.5 kA/m (1200 Oe) necessary for a magnetic recording medium, annealing treatment must be performed at a temperature equal to or higher than a transformation temperature at which the nanoparticles are transformed from a disordered phase to an ordered phase.
Metal nanoparticles prepared by a liquid phase method are generally dispersed in a liquid with an organic dispersant or a polymer. When the dispersion including the nanoparticles is applied to a support and the coating layer is heated at a temperature equal to or higher than the transformation temperature in an inert gas to form a magnetic layer, the metal nanoparticles become magnetically hard, whereas the organic dispersant and the polymer are carbonized. Although this carbonization is desirable to make the magnetization among metal nanoparticles independent, it is undesirable for keeping high adhesion between the support and the magnetic layer and high film strength of the magnetic layer. Specifically, the magnetic layer in which the organic dispersant and the polymer are carbonized is easily peeled off and a surface of the magnetic layer is easily scratched.